(462d) Kinetic Probes for Interfacial Hydrogen Identity in Thermo- and Electrochemical Hydrodeoxygenation Catalysis

At the metal-water interface, H₂ dissociatively adsorbs, forming hydrogen atoms (H···M) which interconvert with hydronium-electron pairs (H₃O⁺···M^-) in the Volmer reaction with distributions that depend on the electrostatic potential φ.^[1,2] Understanding the hydrogen identity at this interface is critical for hydrodeoxygenation reactions, but little is known about the precise distribution of H₃O⁺···M^- and H···M during catalysis, especially since these reactions can occur thermochemically (feeding H₂) or electrochemically (applying cathodic overpotentials). Here, we connect thermo- and electrocatalytic hydrodeoxygenation by introducing selective probe reactions for the two identities: phenol hydrogenation to cyclohexanone and cyclohexanol (HYD), catalyzed by H···M; and electrophilic H-D substitution of phenol-d₆ (HD), catalyzed by H₃O⁺···M^-. HYD rates on Pt surfaces are identical for thermo- and electrocatalysis with two H···Pt coverage regimes: Tafel slopes of -32 mV decade^-1 and 1^st order H₂ dependencies (0.1–0.5 bar H₂) above -370 mV vs SHE, and 0^th order dependencies on P_H2 (0.5–11 bar H₂) or φ below. In tandem, HD rates continually decrease with decrease φ, due to a stronger electrostatic stabilization of H₃O⁺···Pt^- initial states relative to [Pt^-···H₃O⁺···C₆L₅O(L)]^‡ transition states. Taking the HD-to-HYD rate ratio normalizes the changing coverage regimes, depending monotonically on φ from -310 to -410 mV vs SHE, reflecting the sensitivities of elementary rate constants and Volmer equilibrium constant to the interfacial potential. This correlation is nearly identical for various transition metals (Pt, Rh, Ru, and Pd), despite a 100-fold difference in HYD turnover rates (6×10^-2 to 4×10^-4 s^-1, 1 bar H₂ at open circuit), implying that the H₃O⁺···M^- and H···M distribution is independent of metal identity. Taken together, these findings given atomistic insight to the hydrogen identity at metal-water interfaces during hydrodeoxygenation catalysis.

[1] T. S. Wesley, Y. Román-Leshkov, Y. Surendranath, ACS Cent. Sci. 2021, 7, 1045.

[2] J. Shangguan et. al., J. Catal. 2022, 408, 179.